drm/i915: Release connector iterator on a digital port conflict.

Hitting the failure path through check_digital_port_conflicts triggers:

================================================
WARNING: lock held when returning to user space!
4.16.0-rc1-CI-kasan_1+ #1 Tainted: G        W
------------------------------------------------
kms_3d/1439 is leaving the kernel with locks still held!
1 lock held by kms_3d/1439:
 #0:  (drm_connector_list_iter){.+.+}, at: [<000000003745d183>] intel_atomic_check+0x1d9d/0x3ff0 [i915]

Rearrange the code to have a single exit path through the unlock.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180215091425.42364-1-maarten.lankhorst@linux.intel.comReviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -10705,6 +10705,7 @@ static bool check_digital_port_conflicts(struct drm_atomic_state *state)
 	struct drm_connector_list_iter conn_iter;
 	unsigned int used_ports = 0;
 	unsigned int used_mst_ports = 0;
+	bool ret = true;
 
 	/*
 	 * Walk the connector list instead of the encoder
@@ -10739,7 +10740,7 @@ static bool check_digital_port_conflicts(struct drm_atomic_state *state)
 
 			/* the same port mustn't appear more than once */
 			if (used_ports & port_mask)
-				return false;
+				ret = false;
 
 			used_ports |= port_mask;
 			break;
@@ -10757,7 +10758,7 @@ static bool check_digital_port_conflicts(struct drm_atomic_state *state)
 	if (used_ports & used_mst_ports)
 		return false;
 
-	return true;
+	return ret;
 }
 
 static void